Thyroxine 5-deiodinase (, diiodothyronine 5'-deiodinase, iodothyronine 5-deiodinase, iodothyronine inner ring monodeiodinase, type III iodothyronine deiodinase) is an enzyme with system name acceptor:3,3',5'-triiodo-L-thyronine oxidoreductase (iodinating). This enzyme catalyses the following chemical reaction